Cold sensation is derived from activation of the somatosensory system by a cold stimulus. Calcium imaging and patch clamp experiments in dissociated trigeminal and dorsal root ganglia neurons have revealed that cold stimuli induced calcium influx, suggesting the direct opening of a calcium-permeable ion channels by cold (Thut et al., 2003; Reid, 2005). A recently cloned non-selective cation channel, TRPM8 (transient receptor potential melastatin 8) or trp-p8 (identified as a prostate-specific gene, up-regulated in prostate cancer and other malignancies, (Tsavaler et al., 2001)) is activated by cold stimulus of 10 to 24° C. temperature (McKemy et al., 2002; Peier et al., 2002). In addition, TRPM8 is also activated by compounds that elicit cool sensation such as menthol, icilin (AG-3-5) (McKemy et al., 2002), and the endogenous lipid PIP2 (Rohacs et al., 2005). Correlating with the cold sensitivity of both A delta and C-fibers, TRPM8 is highly expressed in sensory neurons of the trigeminal and dorsal root ganglia (McKemy et al., 2002; Peier et al., 2002; Thut et al., 2003). TRPM8 is also expressed in nerve fibers innervating urinary bladder in guinea pigs (Tsukimi et al., 2005) and humans (Mukerji et al., 2006) and believed to contribute to the bladder hypersensitivity.
Activation mechanism of TRPM8 by menthol and icilin appears to differ. Icilin requires calcium for robust activation of TRPM8, whereas menthol and cold do not (Chuang et al., 2004). Typically, activation by all these agonists follows a period of calcium-dependent desensitization. The domain swap analysis of chicken and rat TRPM8 and further mutational studies revealed that determinants of icilin sensitivity map to a region of TRPM8 that corresponds to the capsaicin binding site in TRPV1 transmembrane domain 3 to 4 region (Chuang et al., 2004).
Cold allodynia and mechanical hyperalgesia are associated with neuropathic pain in humans and in rodent models of neuropathic and chemotherapy-induced pain. TRPM8 is shown to mediate the analgesia by agonists such as menthol and icilin (by desensitization of the receptor) during experimental neuropathic pain in rodents (Proudfoot et al., 2006). Further, attenuation of cold sensation and cold allodynia after chronic constriction injury model of neuropathic pain in TRPM8 knockout mice (Colburn et al., 2007; Dhaka et al., 2007) suggests that antagonists of TRPM8 may be considered as pain therapeutics for chemotherapy-induced pain, neuropathic pain and bladder disorders.
Mint oil that contains menthol, an agonist of TRPM8 has been reported to alleviate pain in post-herpetic neuralgia (Davies et al., 2002), a neuropathic pain condition. Furthermore, oral or intracerebroventricular injection of menthol decreased nociceptive responses to hot-plate test and acetic acid-induced writhing in mice (Galeotti et al., 2002). These responses are believed to be mediated by the activation and desensitization of the TRPM8. These observations and the knockout mice studies indicate that TRPM8 modulation by antagonists might be beneficial for patients experiencing neuropathic pain.